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Thread: Calculating chain length

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    Default Calculating chain length

    I know this is a quite discussed topic, but I am having some doubts I can't resolve checking old threats so it may deserve a new thread.
    For the riding I'm doing nowadays I needed lower gear, so I'm going from 52/36 12/29 with short cage derailleur to 50/34 11/32 with long cage derailleur, although I will alternate the 11/32 cassette with the 12/29.
    For calculating the chain length I used several online and app calculators, but I get very confused for a couple of reasons:

    1) Every calculator seems to ignore if the derailleur is short or long cage. Only takes in consideration big chainring, big sprocket and chainstay length (420mm in my bike). Is that right? Can I keep both cassette and crankset, change derailleurs and keep using the same chain length?

    2) According to every calculator, my original chain (on 52/36, 12/29 short cage) was two links short (110 calculated, 108 installed). It was originally installed like this by the mechanic that mounted the bike and I kept that chain length trough changes because always changed well and allowed cross-chain. For my new configuration, 50/34, 11/32 long cage, the calculators estimate a 110 link chain. Should I assume than using 108 links will work fine again? Perhaps I'm trying to be cheap, but I have two 108 link chains in perfect condition that I would hate to dispose, but at the same time I don't want to run intro trouble by installing a too-short chain.

    3) If I follow calculator instructions (that is, 110 link chain) and keep the long cage derailleur, can i switch between my cassettes (12/29 and 11/32) without changing chain, or should I use a correct-length one with each cassette (110 links for 11/32 and 108 links for 12/29)?

    Thanks!

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    Default Re: Calculating chain length


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    Default Re: Calculating chain length

    The algorithm you are using is automating measuring big/big plus two

    Generally this will work if your derailleur is in spec. Running a short cage over maxed, or too small a cassette on a mid/long cage can present issues.

    If I’m worried I mock up the smaller gear (small/small), and the biggest to see If the chain will overtax the derailleur. When in doubt preserve big/big combo as useable.

    Personally I wouldn’t trust a calculator for something like this, mock it up and use your judgement.

    Sizing for a road bike is pretty straightforward, mountain bikes are harder with how much suspension path can influence chain stay length. On some frames (ie yeti sb150) the chain looks way too long in the 10t, but shortening will lock the derailleur out at anything over 60% compression in the 51t. In this case you risk not having enough chain tension in the 10t, as having a rider not shift for a small compression while in the 51t and blowing up the derailleur is much worse.


    In your case I’d do 34/11, and count where the derailleur starts adding tension. Put the bike in 50/32 combo and see if your choice will bind up. If it will go longer as you are likely to use 50/32, while very unlikely to use 34/11

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    Default Re: Calculating chain length

    Caveman approach (warning) I use a 6" long piece of coat hanger bent at both ends. Wrap the chain in the max configuration you intend to abuse, stab the coat hanger into whatever links needed such that the ends of the chain meet and Bob's your uncle.
    Last edited by Too Tall; 08-12-2020 at 08:34 PM.

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    Default Re: Calculating chain length

    Life was so easy back in the Nuovo Record days.
    Mark Walberg
    Building bike frames for fun since 1973.

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    Default Re: Calculating chain length

    Quote Originally Posted by andrew flowers View Post
    The algorithm you are using is automating measuring big/big plus two

    Generally this will work if your derailleur is in spec. Running a short cage over maxed, or too small a cassette on a mid/long cage can present issues.

    If I’m worried I mock up the smaller gear (small/small), and the biggest to see If the chain will overtax the derailleur. When in doubt preserve big/big combo as useable.

    Personally I wouldn’t trust a calculator for something like this, mock it up and use your judgement.

    Sizing for a road bike is pretty straightforward, mountain bikes are harder with how much suspension path can influence chain stay length. On some frames (ie yeti sb150) the chain looks way too long in the 10t, but shortening will lock the derailleur out at anything over 60% compression in the 51t. In this case you risk not having enough chain tension in the 10t, as having a rider not shift for a small compression while in the 51t and blowing up the derailleur is much worse.


    In your case I’d do 34/11, and count where the derailleur starts adding tension. Put the bike in 50/32 combo and see if your choice will bind up. If it will go longer as you are likely to use 50/32, while very unlikely to use 34/11
    I wouldn't trust a formula either.

    It's quick and easy enough to physically measure big/big plus 2 anyway. Furthermore, considering the time saved by not having to count links, and recount just in case, after the formula spits out some number, the total time spent sizing the chain would be much shorter just doing it instead of calculating it.

    I also agree that big/big plus 2 doesn't always work, as I found out on my DA R9100 series equipped bike where it resulted in the big/big combo binding up. My Di2 setting would not allow that combo to occur, but I added 2 more links just in case.
    Chikashi Miyamoto

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    Default Re: Calculating chain length

    I have had more problems with the big big and a compact crank of 34 or less looking good and then the little little having more slack than the rear derailleur can handle. Since I am more likely to use the 32/32 than a 48 by 32 I cheat on the optimal chain being a bit tight on the big big and the small small being optimal. Clear as mud eh?

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    Default Re: Calculating chain length

    Quote Originally Posted by Moke View Post
    I have had more problems with the big big and a compact crank of 34 or less looking good and then the little little having more slack than the rear derailleur can handle. Since I am more likely to use the 32/32 than a 48 by 32 I cheat on the optimal chain being a bit tight on the big big and the small small being optimal. Clear as mud eh?
    What's clear is that one cannot trust a formula!

    It sounds like the Campagnolo method works for a situation like yours: Chain Length Sizing: Campagnolo(R) | Park Tool
    Chikashi Miyamoto

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    Default Re: Calculating chain length

    My unhelpful two cents based on installing ten thousand chains... Pros don’t calculate chain length. They just make sure you have the right drivetrain parts to begin with, and install a chain at the right length.

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    Default Re: Calculating chain length

    Yeah that more succinctly says what I meant. It is a problem solve like anything else on a bike. Edge cases make things confusing. You pay a pro because they have encountered more edge cases than you, and possess the experience than comes with it

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    Default Re: Calculating chain length

    As a certified amateur wrench, Big-big plus 2 has never let me down. Shimano has that recommendation in their install guides as well in case you wanted the official version...

    This is for road setups...
    Last edited by guido; 08-14-2020 at 07:39 PM.
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    Default Re: Calculating chain length

    Quote Originally Posted by guido View Post
    As a certified amateur wrench, Big-big plus 2 has never let me down. Shimano has that recommendation in their install guides as well in case you wanted the official version...
    Only on shadow plus setups prior to the new microspline mtb stuff. This is to help avoid sizing issues that can arise with how much chain tension changes with b gap adjustment for the much wider cassette ranges shadow plus accommodates

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    Default Re: Calculating chain length

    Last I checked there were four? sizing protocols for different generations of drivetrains on s-tech

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    Default Re: Calculating chain length

    Quote Originally Posted by andrew flowers View Post
    Only on shadow plus setups prior to the new microspline mtb stuff. This is to help avoid sizing issues that can arise with how much chain tension changes with b gap adjustment for the much wider cassette ranges shadow plus accommodates
    Sorry I'll add the caveat that my rule was for road... Which is what the OP was asking about...
    Last edited by guido; 08-14-2020 at 07:40 PM.
    Guy Washburn

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    Default Re: Calculating chain length

    Quote Originally Posted by guido View Post
    Sorry I'll add the caveat that my rule was for road... Which is what the OP was asking about...
    Yes, shadow plus road

    If someone was running a dura ace 9000 setup shimano recommends a different method

    Again it’s stupid and confusing

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    Default Re: Calculating chain length

    Quote Originally Posted by Chik View Post
    What's clear is that one cannot trust a formula!

    It sounds like the Campagnolo method works for a situation like yours: Chain Length Sizing: Campagnolo(R) | Park Tool
    I followed that video. Went first with 112 links, but the chain didn't have enough tension. As I had a 108 link chain I could check that was too short, so 110 was the right length (despite having a bit more gap than the 15mm recommended in the video) Ideal would have been 111, but not possible. So at the end, the calculators were right. All the gears work fine, even big-big in case of a mistake, and I learned a lot during the process. Paying for a pro is the easy way but if I do so I would never learn.

    Thanks to everyone for your help!

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    Default Re: Calculating chain length

    Quote Originally Posted by Too Tall View Post
    Caveman approach (warning) I use a 6" long piece of coat hanger bent at both ends. Wrap the chain in the max configuration you intend to abuse, stab the coat hanger into whatever links needed such that the ends of the chain meet and Bob's your uncle.
    Stop complicating this thread with your simplicity.
    Bill Fernance
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